Temporal Light Artifacts (Flicker + Stroboscopic Effect) Dr. Jim Gaines August 1, 2016 Outline Introduction to Temporal Light Artifacts (TLA) Measurements of TLA Standards 3 Definition: Temporal Light Artifacts (TLA) Flicker Perception of visual unsteadiness induced by a light No motion stimulus whose luminance or spectral distribution fluctuates with time, for a static observer in a static environment. ~0-80Hz Stroboscopic Effects Object Change in motion perception induced by a light motion stimulus whose luminance or spectral distribution fluctuates with time, for a static observer in a non- static environment. ~80Hz-2kHz Phantom Array Eye motion Perception of a spatially extended series of light spots when making a saccade (image transition across the retina) across a light source that fluctuates with time ~80Hz-2kHz Definition: Temporal Light Artifacts (TLA) or or What is the problem with TLA? May cause eye strain or headaches • May impair visual or cognitive performance • Distracting • May trigger medical conditions (in severe cases) • Interferes with optical equipment (cameras, bar code • readers, etc.) Could slow adoption of LED lighting due to perceived • poor performance Flicker is sometimes desirable! Sunlight through trees • Reflections off of water • Campfires, candles • Motion pictures • Emergency vehicles • Attention-getting signage • Entertainment • (Although TLA in general lighting probably is bad…) Why do LEDs flicker? • They don’t! (inherently…) They faithfully reproduce light based on the amount of • current flowing through them ElecEtlreiccatrl odnisst oinrtion in Light out Optical distortion out Sources of TLA 1. Source voltage changes (noise) 2. Externally coupled noise sources 3. Dimmer phase angle instabilities (when dimming) 4. Driver instabilities 5. Driver (intended) operation 4 AC Power 3 1 PWM 100% 5 2 ON ON ON ON 0% T Current flicker metrics Simple • Percent Flicker – Flicker Index – Complex • RPI LRC ASSIST – IEC P – st SVM – IEEE 1789 – Percent Flicker (or % Modulation, or Modulation Depth) Easy to understand • Easy to calculate • Assumes periodic waveform • Does not Does not account for frequency • correspond to Does not account for wave shape • human perception! • 𝐴−𝐵 One Cycle 𝑃𝐹 = 100% × 𝐴+𝐵 A (maximum) A-B Light Intensity Time B (minimum)